<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>

    <link rel="stylesheet" href="css/bootstrap.min.css">
    <script src="js/jquery-3.4.1.min.js"></script>
    <script src="js/bootstrap.bundle.min.js"></script>
    <script src="js/bootstrap.min.js"></script>

</head>

<body>

    <nav class="navbar navbar-expand-lg navbar-light bg-light">
        <a class="navbar-brand" href="#">Navbar</a>

        <!-- 
            屏幕尺寸较小的时候，会把导航项收缩到这个折叠菜单中
         -->
        <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent"
            aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
            <span class="navbar-toggler-icon"></span>
        </button>

        <div class="collapse navbar-collapse" id="navbarSupportedContent">
            <ul class="navbar-nav mr-auto">
                <!-- 
                    active：被默认选中
                 -->
                <li class="nav-item active">    
                    <a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
                </li>

                <li class="nav-item">
                    <a class="nav-link" href="#">Link</a>
                </li>
                <!-- 
                    dropdown：下拉菜单
                    dropdown-menu：用于下拉菜单项
                 -->
                <li class="nav-item dropdown">
                    <a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button"
                        data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
                        Dropdown
                    </a>
                    <div class="dropdown-menu" aria-labelledby="navbarDropdown">
                        <a class="dropdown-item" href="#">Action</a>
                        <a class="dropdown-item" href="#">Another action</a>
                        <div class="dropdown-divider"></div>
                        <a class="dropdown-item" href="#">Something else here</a>
                    </div>
                </li>
                <!-- 
                    disable：是超链接失去点击效果
                 -->
                <li class="nav-item">
                    <a class="nav-link disabled" href="#">Disabled</a>
                </li>
            </ul>
            <!-- 
                
             -->
            <form class="form-inline my-2 my-lg-0">
                <input class="form-control mr-sm-2" type="search" placeholder="Search" aria-label="Search">
                <button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button>
            </form>
        </div>
    </nav>

</body>

</html>